In what follows, the circumferential, axial and radial directions of the tire respectively denote a direction tangential to the tread surface of the tire in the direction of rotation of the tire, a direction parallel to the axis of rotation of the tire and a direction perpendicular to the axis of rotation of the tire. “Radially on the inside or, respectively, radially on the outside” means “closer to or, respectively, further away from the axis of rotation of the tire”. “Axially on the inside or, respectively, axially on the outside” means “closer to or, respectively, further away from the equatorial plane of the tire”, the equatorial plane of the tire being the plane that passes through the middle of the tread surface of the tire and is perpendicular to the axis of rotation of the tire.
In general, a tire comprises a tread, intended to come into contact with the ground via a tread surface, the tread being connected by two sidewalls to two beads, the two beads being intended to provide a mechanical connection between the tire and a rim on which the tire is mounted.
A radial aircraft tire more particularly comprises a radial carcass reinforcement and a crown reinforcement both as described, for example, in document EP 1381525.
The radial carcass reinforcement is the tire reinforcing structure that connects the two beads of the tire. The radial carcass reinforcement of an aircraft tire generally comprises at least one carcass layer, each carcass layer being made up of reinforcers, usually textile, coated in a polymeric material of the elastomer or elastomer compound type, the reinforcers being mutually parallel and forming, with the circumferential direction, an angle of between 80° and 100°.
The crown reinforcement is the tire reinforcing structure radially on the inside of the tread and at least partially radially on the outside of the radial carcass reinforcement. The crown reinforcement of an aircraft tire generally comprises at least one crown layer, each crown layer being made up of mutually parallel reinforcers coated in a polymeric material of the elastomer or elastomer compound type. Among the crown layers a distinction is usually made between the working layers that constitute the working reinforcement, usually made up of textile reinforcers, and the protective layers that constitute the protective reinforcement, made up of metal or textile reinforcers and arranged radially on the outside of the working reinforcement. The working layers govern the mechanical behaviour of the crown reinforcement. The protective layers essentially protect the working layers from attack likely to spread through the tread radially towards the inside of the tire. A crown layer, and particularly a working layer, is geometrically characterized by its axial width, which means the distance between its axial ends.
The textile reinforcers of the carcass layers and of the crown layers are usually cords made of spun textile filaments, preferably made of aliphatic polyamide or of aromatic polyamide. The mechanical properties under tension (modulus, elongation and breaking force) of the textile reinforcers are measured after prior conditioning. “Prior conditioning” means the storage of the textile reinforcers for at least 24 hours, prior to measurement, in a standard atmosphere in accordance with European Standard DIN EN 20139 (temperature of 20±2° C.; relative humidity of 65±2%). The measurements are taken in the known way using a ZWICK GmbH & Co (Germany) tensile test machine of type 1435 or type 1445. The textile reinforcers are subjected to tension over an initial length of 400 mm at a nominal rate of 200 mm/min. All the results are averaged over 10 measurements.
An elastomeric material, such as the one used to coat the reinforcers of the carcass layers and of the crown layers, can be mechanically characterized, after curing, by tensile stress/strain characteristics determined by tensile testing. This tensile testing is carried out on a test specimen according to a method known to those skilled in the art, for example in accordance with international standard ISO 37 and under normal temperature (23+ or −2° C.) and relative humidity (50+ or −5% relative humidity) conditions defined by international standard ISO 471. The elastic modulus at 10% elongation of an elastomeric compound, expressed in megapascals (MPa), is the name given to the tensile stress measured for a 10% elongation of the test specimen.
During the manufacture of an aircraft tire and, more specifically, during the step of laying the working reinforcement, a working layer is usually obtained by a circumferential winding in turns or by a circumferential zigzag winding of a strip made up of at least one continuous textile reinforcer coated in an elastomeric compound, on the lateral surface of a building drum. Whether produced by circumferential winding in turns or circumferential zigzag winding, the working layer is then made up of the juxtaposition of a width of strip for each turn of winding.
A circumferential winding in turns means a winding in the circumferential direction of the tire and in a helix of diameter equal to the diameter of the building drum on which the strip is laid and with a mean angle of between 0° and 5° with respect to the circumferential direction. The working layer thus obtained by winding in turns is said to be circumferential because the angle of the textile reinforcers of the strip, one parallel to the next, formed in the equatorial plane with the circumferential direction, is between 0° and 5°.
Circumferential zigzag winding means a winding in the circumferential direction of the tire and with a periodic curve, which means to say one formed of periodic waves oscillating between extrema. Winding a strip with a periodic curve means that the mid-line of the wound strip, defined as being the line equidistant from the edges of the strip, coincides with the periodic curve. The peak-to-peak amplitude between the extrema of the periodic curve thus defines the axial width of the working layer, namely the distance between the axial ends thereof. The period of the periodic curve is usually between 0.5 times and 3 times the circumference of the building drum on which the strip is laid. The periodic curve is also characterized by the angle it forms, or more specifically by the angle that the tangent to the curve forms, in the equatorial plane of the tire with the circumferential direction of the tire, and by a radius of curvature, at the extrema of the periodic curve. For a conventional zigzag winding, the angle of the periodic curve, which corresponds to the angle formed by the textile reinforcers, one parallel to the next, that make up the strip, is generally between 5° and 35° with respect to the circumferential direction. A circumferential zigzag winding means that the working layers have to be assembled in pairs, a pair of working layers constituting a working bi-ply.
A working bi-ply is made up of two working layers radially superposed in the main section, namely in the portion axially on the inside of the two axial ends of the working bi-ply, and by more than two working layers radially superposed at the axial ends thereof. The additional number of additional working layers in the radial direction, at the axial ends, in comparison with the two working layers in the main section is referred to as the axial end additional thickness. This axial end additional thickness is generated by the crossings of strip at the extrema of the periodic curve. A working reinforcement is thus made up of the radial superposition of several working bi-plies. Such a working reinforcement comprising working layers obtained by circumferential zigzag winding of a strip has been described in documents EP 0540303, EP 0850787, EP 1163120 and EP 1 518 666.
In the case of circumferential zigzag winding, it is known that the axial end additional thicknesses of the working bi-plies are sensitive to the onset of endurance damage, such as cracks which may evolve into significant degradation of the working reinforcement and, therefore, reduce the life of the tire. These cracks may appear at the internal interfaces of an axial end additional thickness of a working bi-ply, or at the interface between the axial end additional thicknesses of two adjacent working by-plies.
Document FR 2910381 describes an aircraft tire that can be used at high speed, under heavy load, while having a low weight. Such a tire comprises a working reinforcement comprising at least one working layer wound in turns radially on the inside and at least two working layers wound in a zigzag radially on the outside, at least one working layer wound in turns being wider than the zigzag working layers.